Patent GB-554471-A (GB)
Title: Improved method of producing gears for transmitting non-uniform motion and gears produced thereby
IPC: B23F17/00
Filed: 1941-12-31
Abstract: 554,471. Gear-cutting. GLEASON WORKS. Dec. 31, 1941, No. 16855. Convention date, Dec. 31, 1940. [Class 83 (iii)] [Also in Group XXIV] Spur or bevel gears for transmitting non-uniform motion, such as those used in the differentials of tractors are generated by producing a cyclic variation in the rolling movement between the tool and the blank, the period of the cycle corresponding to rotation of the work through one pitch. When applied to bevel gears, the teeth preferably are of longitudinally curved shape with zero spiral angle. The variation in velocity ratio or leverage of gears of this type is usually obtained by modification of the tooth profile from the involute 13 (dotted line in Fig. 1) to a curve such as 14 having less curvature than the involute between 17 and 16 and a greater curvature at the part 141. According to the invention the modified profile 12 lies entirely within the involute and the maximum curvature is at the central portion 18, 19 of the profile. The line of action while the part 18-19 of mating gears is in contact is from 20-21 while the other parts give lines of action 21, 23 and 20, 22. The profiles may be such that the line of action forms a smooth curve 23, 21, 20, 22 with a point of inflexion at 15. The normals to the tooth profile at the points of contact intersect the line of centres 25 of mating gears at spaced points corresponding to different leverage ratios. One type of mating pair according to the invention is shown in Fig. 3. The tooth surfaces of both gear 30 and pinion 31 may be generated conjugate to a straight-sided basic rack 35 by varying the rolling motion during generating in a manner corresponding to the variation in velocity ratio of the gears rotating in mesh. In other words, the basic rack 35, Fig. 4 (or crown wheel in the case of bevels) may be regarded as having an undulatory pitch surface 55, the undulations corresponding to the pitch of the teeth and having an extent dependent on the amount of shift of the instantaneous axis 48, Fig. 3, of the gears in mesh. Both sides of the gear teeth may be generated simultaneously. When bevel gears are cut they are preferably curved and have a depth taper from end to end. In order to avoid bias bearing between such gears the gears are cut with a face-mill cutter having spherical tooth profiles and the gears are cut conjugate to a crown gear 95, Fig. 6, having spherical side tooth surfaces. The inside and outside spherical cutting edges are centered at 98, 99 respectively. In the position shown the torque ratio has one of its extreme values and the teeth are not symmetrical about the line of centres 94. As before the basic crown gear has an undulating pitch surface. In Fig. 8, which shows the setting for generating both sides of the teeth of a pinion 116 simultaneously by means of a spherical edged face-mill cutter 115 the cradle axis 125 passes through the intersection 123 of the pinion and gear axes and is perpendicular to the root plane 130 of the gear. During generation, the work is rotated on its axis with uniform velocity and the cutter rolled round the cradle axis at a non-uniform velocity. Lengthwise mismatch of the mating tooth. surfaces may be produced by cutting the convex tooth sides of one gear member to a smaller radius than the concave sides of the mating member. Profile mismatch may also be obtained by variation of the position of the cradle axis.